1. Field of the Invention
The present invention relates to a polarization-splitting coupler used for an optical communication and optical transmission and a method of manufacturing the same, and more specifically, to a polarization-splitting fiber coupler having a polarized light coupling portion formed by heating, fusing and drawing a plurality of optical fibers.
2. Description of the Related Art
A polarization-splitting fiber coupler made by fusing fibers is widely used because it has such an advantage as a less amount of inserting loss, low manufacturing cost and the like. In particular, since the polarization-splitting fiber coupler has a polarization-keeping characteristics, it is used for the coherent optical transmission and the like of an optical multiplex transmission.
Conventionally, two methods are available to make a polarization-splitting fiber coupler: in a first method, two non-polarization fibers are arranged and a portion thereof is heated, fused and drawn in the axial direction thereof while optimizing an aspect ratio; and in a second method, two polarization-maintaining fibers 2, 3 are arranged after having been adjusted by axially rotation so that the planes of polarization thereof face to the same direction and a portion thereof is heated and fused by an oxyhydrogen burner 4 to draw the fibers 2, 3 in the axial direction thereof so as to form a fused/drawn portion 1a for the manufacture of a polarization-splitting fiber coupler 1, as shown in FIG. 1(A).
In the above fused/drawn portion 1a, it is found that the state that a clad can be regarded as a core is achieved as a result that a new coupling mode is made by an evanescent light leaked from the core, whereas polarization separation/coupling characteristics greatly depend upon the diameter and configuration of the clad. More specifically, it is contemplated that a mode phase difference is produced between the X-polarized light and Y-polarized light of the clad portion of the fused/drawn portion 1a and the mode phase difference is greater than the mode phase difference therebetween produced by the portion of the polarization-maintaining fibers. Further, since the mode phase difference is different depending upon the wavelength of a light, the polarization separation/coupling characteristics of the polarization-splitting fiber coupler has a wavelength-depending property.
Incidentally, in the first method, although the non-polarization fibers must be heated and fused while optimizing the aspect ratio, the optimization of the aspect ratio depends upon the perception and experience of a manufacturer and thus the aspect ratio is not always stably kept to an optimum value. Under such a circumstance, the conventional polarization-splitting fiber coupler is drawn and made so that a phase difference between an X-polarization light and a Y-poralization light is .pi./2 and thus the fused/drawn portion thereof must be heated and drawn up to 20 cycle days. More specifically, generally speaking when the fused/drawn portion is heated and drawn, the degree of polarization separation thereof is cyclically changed, it must be continuously heated and drawn until the degree of polarization separation of 100% arises 20 times. As a result, the fused/drawn portion has a considerably increased length L. When the length L of the fused/drawn portion is increased, the polarization state of the fused/drawn portion is liable to be unstable and thus polarization separation/coupling cannot be stably executed.
Further, the polarization-splitting fiber coupler made by the first method has a problem in that since the fibers does not have a polarization keeping function by themselves, the polarization state thereof is made unstable before a polarized light reaches the fused/drawn portion and thus the polarization separation/coupling cannot be stably executed, in addition to that the optimization of the aspect ratio is difficult.
On the other hand, FIG. 1(B) shows the wavelength characteristics of the conventional polarization-splitting fiber coupler 1 made by the second method, wherein the ordinate shows a loss of the X- and Y-polarized lights at the light emitting end C of the polarization-maintaining fiber 3 and the abscissa shows a wavelength of a light in the case that the X-, Y-polarized lights are incident from the incident end A of the polarization-maintaining fiber 2 and the X-polarized light is taken out from the light emitting end B of the polarization-maintaining fiber 2 and the Y-polarized light is taken out from the light emitting end C of the polarization-maintaining fiber 3. In this conventional polarization-splitting fiber coupler 1, a wavelength region .DELTA..lambda. where a degree of separation between the X-polarized light and the Y-polarized light exceeds 10 dB is about 30 nm.
Further, since the conventional polarization-splitting fiber coupler 1 is composed of the polarization-maintaining fibers 2, 3 having a short length of 1-2 m, it has a weak polarization selection property and thus the X-, Y-polarized lights each having a slightly shifted plane of polarization reach the fused/drawn portion 1a so that a sufficient degree of polarization separation/coupling cannot be obtained. That is, a degree of polarization separation in the conventional polarization-splitting fiber coupler 1 is about 13 dB at best. Further, an inserting loss may be increased or a degree of polarization separation may lowered due to the shift of the planes of polarization of the polarization-maintaining fibers 2, 3 and the polarization-maintaining fiber (not shown) of a transmitting portion connected thereto at the junction where they are connected to each other.